Method for reducing voltage drops in symmetric array architectures

ABSTRACT

A symmetric segmented array has select transistors and column select transistors. At least one of the select and/or column select transistors is a low threshold voltage device. Alternatively, at least one select transistor and/or column select transistor of the array has a channel length shorter than a standard channel length of a process.&lt;/PTEXT&gt;

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patent application, Ser. No. 09/348,720, filed Jul. 6, 1999 now a U.S. Pat. No. 6,285,574, which is a continuation of U.S. application 08/989,690 filed Dec. 12, 1997, now a U.S. Pat. No. 5,963,465 and incorporated in its entirety by reference herein.

FIELD OF INVENTION

The present invention relates to memory array architectures generally and to symmetric memory array architectures in particular.

BACKGROUND OF THE INVENTION

Memory arrays are well known in the art and comprise matrices of memory cells organized into rows and columns. Each memory cell comprises a source, a drain and a gate, each of which has to receive voltage in order for the cell to be accessed. Columns of sources and columns of drains are connected together by bit lines while rows of gates are connected together by word lines. To activate a cell, one drain bit line, one source bit line and one word line must receive voltage.

A standard memory array architecture consists of one metal line on each column, periodically connected to the underlying bit line via a contact. The contact typically is large and is present within the memory array area. The word line is typically of lower resistance and its contact is located outside of the memory array area. There is typically a common source line for a plurality of memory cells. Furthermore, the metal lines are themselves quite thick. Typically, the distance between bit lines is defined by the width of either or both of the metal lines and the contacts, where the contacts are typically wider than the metal lines.

Various memory array architectures are known which reduce the size of the memory array area by reducing the number of contacts and/or metal lines. In virtual ground architectures, the common ground line is eliminated. Instead, the drain of one cell serves as the source for its neighboring cell. Bit lines are continuous diffusions with a contact to the metal lines every X (8, 16, 24, 32, 64, 128, etc.) cells to reduce resistance. The gain in area is up to 40% due to the reduced number of contacts and the elimination of the common source line.

To further reduce array size, the alternate metal, virtual ground architecture (AMG), described in U.S. Pat. No. 5,204,835, has two bit lines per metal line. Typically, in the AMG architecture, the cell size is close or equal to the minimum feature size possible for the cells.

Standard virtual ground architectures access every cell symmetrically (i.e. every bit line receives voltage directly from a metal line). The AMG architecture, which is more compact than standard virtual ground architectures, directly provides voltage to the metalized bit lines but indirectly provides voltage to the segmented, non-metalized bit lines. As a result, the voltage on an activated non-metalized bit line (which is provided through n-channel select transistors) is lower than the voltage on a simultaneously activated metalized bit line. Furthermore, n-channel transistors are not good at passing the high voltages needed for programming.

The non-symmetry of the AMG architecture makes it difficult to use with a nitride read only memory (NROM) array which stores two bits in each NROM cell. Such a cell is described in Applicant's copending application Ser. No. 08/905286, filed Aug. 1, 1997 entitled “Two Bit Non-Volatile Electrically Erasable and Programmable Semiconductor Memory Cell Utilizing Asymmetrical Charge Trapping” and assigned to Saifun Semiconductors, the same assignee as for this application, whose disclosure is incorporated herein by reference. The two bits in a cell are located on each side of the cell and each bit is accessed by voltages on the two neighboring bit lines of the cell. Accordingly, the cell requires that its two neighboring bit lines receive equivalent amounts of voltage thereby to read both bits equally (although not simultaneously).

Some architectures segment the bit lines. Each row of segmented bit lines is called a “block” and each block typically includes block select transistors to activate only one block at a time. This is particularly important for FLASH electrically erasable, programmable, read only memory (FLASH EEPROM) arrays which pass high voltages along the bit lines during programming and erase operations. During programming, the bit line voltages disturb the unselected cells.

To reduce the total time the programming voltage disturbs the cells, the bit lines are segmented into small blocks.

SUMMARY OF THE PRESENT INVENTION

An object of the present invention is to provide an improved symmetric segmented array having a minimum voltage drop therein. It is noted that such an array has column select transistors controlling metal bit lines which, in turn, are connected to select transistors which control diffusion bit lines.

In accordance with a preferred embodiment of the present invention, at least one select transistor and/or column select transistor of the array is a low threshold voltage device.

Alternatively, in accordance with a preferred embodiment of the present invention, at least one select transistor and/or column select transistor of the array has a channel length shorter than a standard channel length of a process.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:

FIG. 1 is a schematic illustration of an arrangement of bit lines in a memory array, in accordance with a preferred embodiment of the present invention;

FIG. 2 is a schematic illustration of an exemplary arrangement of metal lines over the bit lines of FIG. 1 in accordance with the present invention;

FIG. 3 is a schematic illustration of a dual bit cell of tho type disclosed in copending application Ser. No. 08/905286, filed Aug. 1, 1997 entitled “Two Bit Non-Volatile Electrically Erasable and Programmable Semiconductor Memory Cell Utilizing Asymmetrical Charge Trapping” and assigned to Saifun Semiconductors, the same assignee as for this application;

FIGS. 4A, 4B, 4C and 4D are illustrations of one exemplary layout of the architecture of FIG. 2;

FIG. 5 is an illustration of an alternative layout of the architecture of FIG. 2;

FIG. 6 is an illustration of a further alternative layout of the architecture of FIG. 2;

FIG. 7 is a schematic circuit diagram illustration showing the flow of current when accessing a particular memory cell;

FIG. 8 is a schematic circuit diagram illustration, similar to FIG. 7, showing the voltage levels on the metal bits lines prior to accessing a particular memory cell; and

FIGS. 9A and 9B are graphical illustrations of the rise of voltage on a metal bit line and a diffusion bit line for the drain and source, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 1, which schematically illustrates the arrangement of bit lines within the memory array of the present invention.

The memory array is divided into alternating cell areas and select areas, where the cell areas have cell bit lines and the select areas have contact bit lines. Four cell areas are shown, labeled A, B, C and D. A typical memory area will have many more cell areas than shown.

The cell bit lines are organized into groups, each group having four columns. Two groups i and i+1 are shown, each group having columns labeled 1, 2, 3, and 4. The contact bit lines come in two flavors, even and odd (labeled E and O, respectively) and are found in alternate select areas. Every group of cell bit lines is associated with one even and one odd contact bit line. Thus, FIG. 1 shows even select areas between cell areas A and B and between cell areas C and D. Both even select areas have two even contact bit lines E_(i) and E_(i+1). FIG. 1 shows an odd select area between cell areas B and C which has two odd contact bit lines O_(i) and O_(i+1).

As can be seen, there is one contact bit line per group of four cell bit lines per select area and there are two types of select areas. FIG. 1 shows that the even contact bit lines E are associated with column 3 of each group while the odd contact bit lines are associated with column 2 of each group. As will be shown in the layouts of FIGS. 4 and 5, the contact bit lines are not necessarily formed within column 2 or 3 but rather within a four column portion of the select area.

As shown in FIG. 2 to which reference is now made, one contact is formed on each contact bit line and is labeled with an apostrophe (′). For example, the contacts of contact lines E_(i) are labeled E_(i)′. Since there only one contact bit line per four cell bit lines, the contact can be as large as necessary without generally affecting the distance between cell bit lines.

There are two metal lines MO and ME per group of cell bit lines, where even metal lines ME connect together the even contacts of the group and odd metal lines MO connect together the odd contacts of the group.

There are four select transistors connected to each contact bit line O or E and each type of select transistor is controlled by its own select line SEL. Each select transistor connects its contact bit line to one nearby cell bit line. Thus two of the four select transistors connect to two of the cell bit lines in the group of cell bit lines above the contact bit line. Similarly, the remaining two of the four select transistors connect to two of the four cell bit lines in the group below.

In accordance with a preferred embodiment of the present invention, the select transistors associated with each odd contact bit line O connect only to odd columns and the select transistors associated with each even contact bit line E connect only to even columns. Thus, the four select transistors 2U, 4U, 2L, 4L of each contact bit line E connect to columns 2 and 4 of the upper group of bit lines and to columns 2 and 4 of the lower group of bit lines, respectively. These four select transistors are activated by select lines SEL 2U, SEL 4U, SEL 2L and SEL 4L, respectively. Similarly, the four select transistors 1U, 3U, 1L, 3L of contact bit line O connect to columns 1 and 3 of the upper group of bit lines and columns 1 and 3 of the lower group of bit lines, respectively. They are activated by select lines SEL 1U, SEL 3U, SEL 1L and SEL 3L, respectively.

It will be a appreciated that all four columns of bit lines receive power even though there are only two contact bit lines per group of four cell bit lines. It will further be appreciated that each cell bit line is accessed by only one select transistor.

FIG. 2 indicates that memory cells are located between neighboring cell bit lines, a row of which is accessed with a single word line WL. For clarity, only cells P, Q, R, and S are shown with their word line WLB. The memory cells can be any type of memory cell, such as ROM, EPROM, EEPROM and FLASH EEPROM cells, for example.

The present architecture is particularly suited to memory cells that prefer symmetric accessing. For example, the NROM cell, described in Applicant's copending application Ser. No. 08/902890 filed Jul. 30, 1997, entitled “Non-Volatile Electrically Erasable And Programmable Semiconductor Memory Cell Utilizing Asymmetrical Charge Trapping” and assigned to Saifun Semiconductors, the same assignee as this application, which is incorporated herein by reference, is read in one direction and programmed in the other direction. This cell requires a symmetric architecture such as is shown herein in FIG. 2. Furthermore, the dual bit NROM cell, described in Applicant's copending application Ser. No. 08/905286 filed Aug. 1, 1997, entitled “Two-Bit Non-Volatile Electrically Erasable And Programmable Semiconductor Memory Cell Utilizing Asymmetrical Charge Trapping” and assigned to Saifun Semiconductors, the same assignee as this application which is incorporated herein by reference and shown hereinbelow with respect to FIG. 3, requires a symmetric architecture to read both bits of the cell. The following discussion will assume a single bit cell unless specifically mentioned otherwise.

As will be described herein in more detail, each memory cell is accessed by one select transistor “above” it and one select transistor “below” it, thereby ensuring that the current path from the two active contacts is equal for every memory cell and thus, the resistance associated with the current path to each memory cell is generally equal.

The four exemplary cells in row B are labeled P, Q, R and S, where-cell P is between cell bit lines 1B_(i) and 2B_(i), cell Q is between cell bit lines 2B_(i) and 3B_(i), cell R is between cell bit lines 3B_(i) and 4B_(i) and cell S is between cell bit lines 4B_(i) and 1B_(i+1). An exemplary current path through cell Q moves either from cell bit line 2B_(i) to cell bit line 3B_(i) or in the reverse direction. In either case, cell bit lines 2B_(i) and 3B_(i) must receive power from their respective metal lines ME_(i) and MO_(i). Even metal line ME_(i) provides a first voltage to even contact E_(i)′ which is connected to even cell bit line 2B_(i), via select transistor 2L. Thus, select line SEL 2L must be activated. Odd metal line MO_(i) provides power to odd contact O_(i)′ which is connected to odd cell bit line 3B_(i) via select transistor 3U. Thus, select line SEL 3U must be activated.

It is noted that two select transistors are required to provide current to cell Q, one of which is an “upper” select transistor and one of which is a “lower” select transistor. This is true for every memory cell in the array. The two neighboring bit lines for a given cell are accessed from opposite directions and thus, the length of the current path through every cell is generally constant. The result is constant resistance for the current path associated with each memory cell.

Furthermore, since each cell bit line is accessed by only one select transistor, each memory cell is symmetrically accessed.

To access a single bit memory cell, the following must be activated: its word line WL, the two select lines associated with the select transistors which are connected to the cell bit lines of the memory cell, and the metal lines connected to the contacts associated with the activated select transistors. For example, cell Q is accessed by activating word line WLB, select lines SEL 2L and SEL 3U which activate select transistor 2L connected to contact E_(i)′ and select transistor 3U connected to contact O_(i)′, respectively, and metal lines ME_(i) and MO_(i) (which provide power to contacts E_(i)′ and O_(i)′, respectively).

The following table lists the cell bit lines that must receive power in order to access each cell P, Q, R and S of FIG. 2. It also lists the metal lines and select transistors needed to provide power to the listed cell bit lines. The select transistors are activated by activating the lines associated with them. Since the memory cells P, Q, R and S are on a single word line WLB, the word line is not listed though it must also be activated.

TABLE 1 Drain Metal Source Metal Drain Select Source Select Cell Line line Transistor Transistor P MO_(i) ME_(i) 1U 2L Q ME_(i) MO_(i) 2L 3U R MO_(i) ME_(i) 3U 4L S ME_(i)  MO_(i+1) 4L 1U

It is noted that cells P, Q, R and S define the four different types of cells.

Each of these cells is accessed by the two metal lines providing power to their bit lines and by the two upper and two lower select lines which surround their bit lines.

For most memory cells, the metal line associated with the left cell bit line will receive the source voltage for reading, programming (if the cell is programmable) and erasure (if the cell is electrically erasable). For NROM cells which are programmed in the opposite direction from which they are read, the metal line associated with the left cell bit line will receive the source voltage during reading and the drain voltage during programming. The symmetry of the present architecture ensures that the NROM cells will be fully accessible during both reading (forward direction) and programming (backward direction).

The architecture of the present invention can also be utilized for dual bit cells of the type which have one bit on either side of the cell. Such a dual bit cell is described in Applicant's copending U.S. patent application Ser. No. 08/905286, filed Aug. 1, 1997 entitled “Two Bit Non-Volatile Electrically Erasable and Programmable Semiconductor Memory Cell Utilizing Asymmetrical Charge Trapping” and assigned to Saifun Semiconductors, the same assignee as for this application. FIG. 3, to which reference is now made, schematically illustrates the dual bit cell of U.S. patent application Ser. No. 08/905286, filed Aug. 1, 1997. The cell has a single channel 100 between two bit lines 102 and 104 but has two separated and separately chargeable areas 106 and 108. Each area defines one bit. For the dual bit cell of FIG. 3, the separately chargeable areas 106 and 108 are found within a nitride layer 110 formed in an oxide-nitride-oxide sandwich (layers 109, 110 and 111) underneath a polysilicon layer 112.

To read the first bit, stored in area 106, bit line 104 is the drain and bit line 102 is the source. The cell is designed to ensure that, in this situation, only the charge in area 106 will affect the current in channel 100. To read the second bit, stored in area 108, the cell is read in the opposite direction. Thus, bit line 102 is the drain and bit line 104 is the source.

The truth table for reading the bits of cells P, Q, R and S, assuming the cells are dual bit cells, is presented hereinbelow in Table 2. Typical voltages might be: 2.7-3 V on the word line WL (not listed in Table 2), 1.6-2V on the drain metal line, 0V on the source metal line and the standard chip voltage Vcc on the select lines. The subscript 1 refers to the left bit 106 and the subscript 2 refers to the right bit 108 in the cell of FIG. 3.

TABLE 2 Drain Metal Source Metal Cell Line Line Select Lines P1 MO_(i) ME_(i) 1U, 2L P2 ME_(i) MO_(i) 2L, 1U Q1 ME_(i) MO_(i) 2U, 3L Q2 MO_(i) ME_(i) 3L, 2U R1 MO_(i) ME_(i) 3U, 4L R2 ME_(i) MO_(i) 4L, 3U S1 ME_(i)  MO_(i+1) 4L, 1U S2  MO_(i+1) ME_(i) 1U, 4L

The dual bit NROM cell described in Applicant's copending application Ser. No. 08/905286, filed Aug. 1, 1997 stores two bits, as in FIG. 3. However, the dual bit NROM cells are programmed in the opposite direction from which it is read. Thus, if the first bit is read in one direction, it is programmed in the opposite direction.

The truth table for programming the bits of cells P, Q, R and S, assuming the cells are dual bit NROM cells, is presented hereinbelow in Table 3. Typical voltages might be: 6-10V on the word line WL (not listed in Table 3), 5.5-6.5V on the drain metal line, 0V on the source metal line and 10V on the select lines.

TABLE 3 Drain Metal Source Metal Cell Line Line Select Lines P1 ME_(i) MO_(i) 2L, 1U P2 MO_(i) ME_(i) 1U, 2L Q1 MO_(i) ME_(i) 3L, 2U Q2 ME_(i) MO_(i) 2U, 3L R1 ME_(i) MO_(i) 4L, 3U R2 MO_(i) ME_(i) 3U, 4L S1  MO_(i+1) ME_(i) 1U, 4L S2 ME_(i)  MO_(i+1) 4L, 1U

For erasing the data in NROM cells, the word lines of interest (one block or one or more blocks of word lines or the entire array of word lines) must be selected. In addition, for any set of word lines, all of the cell bit lines of the array must be selected while only the four select lines surrounding each block of word lines of interest must be selected. Typically, the word lines receive 0V or −5 to −8V, the bit lines receive 5.5 to 6.5V and the select lines receive 10V.

Reference is now made to FIGS. 4A, 4B, 4C and 4D which provide an exemplary layout for the architecture of FIG. 2. FIG. 4A shows, in expanded view, the isolation and bit line layers for the area around and including one section of select transistors, FIG. 4B shows the same area with the addition of the select lines, FIG. 4C shows the same area with the metal layer and FIG. 4D shows two blocks of the array and three select transistor sections, with the metal lines.

The manufacturing process which utilizes the layout shown herein can be any suitable manufacturing process. For example, it can be the process described in U.S. Pat. No. 5,966,603, assigned to Saifun Semiconductors, the same assignee as this application, whose disclosure is incorporated herein by reference.

The isolation layer 120, shown in white, is typically a field oxide layer grown in the initial stages of manufacturing and provides isolation between neighboring select transistors.

After the isolation layer 120 is produced, a bit line mask 122 is laid down and a bit line implant is performed, producing bit lines wherever there is neither bit line mask 122 nor isolation 120. As can be seen, at the edge of the select area (and marking the edge of the cell area), the bit line mask 122 has a plurality of U shapes (these are marked with reference numerals 122A). The cell bit lines extend within and between the U shapes. FIG. 4A marks columns 3A_(i), 4A_(i) and 1A_(i+1) to 4A_(i+1) above the select area and columns 3B_(i), 4B_(i), and 1B_(i+1) to 4B_(i +1) below the select area. It is noted that the even columns extend into the select area (the odd columns extend into the neighboring select areas above and below the select area shown in FIG. 4A).

The bit line mask 122 also has horizontal select transistor markers, labeled 122B, 122C, 122D and 122E. These markers define the channels of the four select transistors. As can be seen, column 2A_(i+1) extends to transistor marker 122B, column 4A_(i+1) extends to transistor marker 122C, column 4B_(i+1) extends to transistor marker 122D, and column 2B_(i+1) extends to transistor marker 122E. These columns form the sources for the four select transistors 2U, 4U, 4L and 2L, respectively.

The remaining bit line area (labeled 124 and formed into an E shape) is the contact bit line and it forms the drains for the four select transistors, as follows: the upper bar of the E, labeled 124A, forms the drain of the 2U transistor (opposite column 2A_(i+1)), the lower bar of the E, labeled 124B, forms the drain of the 2L transistor (opposite column 2B_(i+1)), and the middle bar of the E, labeled 124C, forms the drains of the 4U and 4L transistors (opposite columns 4A_(i+1) and 4B_(i+1), respectively). As can be seen, the contact E_(i+1)′ is formed within middle bar 124C. FIG. 4A also shows contact E_(i)′.

As shown in FIG. 4B, the select lines are formed from a layer of polysilicon into rows over the channels 122B-122E of the select transistors. FIG. 4B shows the select lines themselves, labeled 126, rather than the mask which forms them. Each select line controls only one type of select transistor.

As can be seen, the vertical line of the contact bit line 124 forms a “feedthrough”, labeled 127 in FIG. 4B, below the select lines to connect the various drains. Furthermore, the contact bit line 124 and the select transistors reside within the pitch of four cell bit lines (columns 1, 2, 3 and 4 of group i+1). The contact E_(i+1)′ is aligned with column 3 although the feedthrough of contact bit line 124 is not. The elements labeled 127′ are feedthroughs of the cell bit lines under the select lines.

FIG. 4C is similar to FIG. 4B but also shows the metal lines of which only metal lines ME_(i), MO_(i+1) and ME_(i+1) are labeled. Only metal lines ME_(i) and ME_(i+1) are connected to their respective contacts E_(i)′ and E_(i+1)′. The other metal line MO_(i+1) continues through to the neighboring select areas (as can be seen in FIG. 4D). FIG. 4D labels six contacts, O_(i)′ and O_(i+1)′ in the first select area 130, E_(i)′ and E_(i+1)′ in the second select area 132 and O_(i)′ and O_(i+1)′ in the third select area 134.

It is noted that select areas 130 and 134 are slightly different than select area 132 (which is shown in FIGS. 4A, 4B and 4C) since they connect to different contacts. However, the principles discussed herein are the same for all select areas.

FIG. 5, to which reference is now briefly made, has a similar layout to that of FIGS. 4A, 4B and 4C except that the bit lines are formed with two implants.

The first implant, marked in FIG. 5 with hatching, is provided in the cell areas to create straight portions 140 of the bit lines and, in the select areas, only in the areas of feedthroughs 127 and 127′. The remainder of the select areas are masked over. In FIG. 5, feedthroughs 127 connect the parts of contact bit line E_(i) and feedthroughs 127′ connect the parts of column 4 of each group. For odd contact bit lines O_(i), the feedthroughs connect the parts of the odd contact bit lines and the parts of columns 3 of each group.

The first mask is not shown in full in FIG. 5; instead, the U shapes 122A are shown but, for clarity only, the shape of the mask within the select areas is not shown.

After the bit lines and feedthroughs are implanted, the gate oxide for CMOS peripheral devices and for the select devices is grown using thermal oxidation. Following the oxidation, the select lines SEL are deposited in the select areas after which a second, n+implant is implanted in the select areas only. This second implant is noted in FIG. 5 with dots. It is noted that the second implant very slightly overlaps the cell area to ensure contact between the cell bit lines and the contact bit lines.

Because the second implant is performed after the select lines SEL are deposited, the second implant is self-aligned to the select lines. Furthermore, since the first implant was only in the areas of the feedthroughs 127 and 127′, most of the areas under the select lines have no n+implant and thus, the select lines define the channels of the select transistors. This is not true for the areas of the feedthroughs 127 and 127′ which provide electrical feedthrough under the select lines SEL.

The combination of the first and second implants produces the cell and contact bit lines, For example, the straight portions 140 of columns 2A_(i+1) and 2B_(i+1), produced by the first implant, electrically connect with “T” portions, labeled 142, of the second implant which is on one side of select lines SEL 2U and 2L, respectively. On the other side of select lines SEL 2U and 2L are portions 144 which, via feedthroughs 127, are electrically connected to the central portion 124C of contact bit line E_(i+1).

Similarly, the straight portion 140 of columns 4A_(i+1) and 4B_(i+1), produced by the first implant, extend electrically under the select lines SEL 2U and SEL 2L, respectively, due to feedthroughs 127′, where they electrically connect with “L” portions 146 of the second Implant. On the other side of select lines SEL 4U and 4L is the central portion 124C of contact bit line E_(i+1).

As shown in FIG. 5, the contact bit line (marked with dots) has the same shape as before since it is located in the portion of the select areas not covered with field isolations 120. However, the channels of the select transistors are now defined by the width and location of the select lines SEL rather than by a bit line mask as in the previous embodiment. This enables the channels to be thinner than in the previous embodiment and it provides self-alignment to the select transistors.

FIG. 6, to which reference is now briefly made, shows an alternative layout for the select areas. In this embodiment, the select transistor markers 122B-122E are vertical The bit and select lines are changed accordingly. Sections 124D, 124E and 124F form one continuous contact bit line 124.

It will be appreciated that any layout which produces an architecture with the same connections as those of FIG. 2 is incorporated within the present invention and that the layouts of FIGS. 4, 5 and 6 are exemplary only.

Reference is made to FIG. 7, which illustrates an exemplary current path through one of the cells in the array of the present invention during programming. As can be seen, to program the cell R, the current flows from a power supply, through a series of transistors in a column multiplexer (YMUX) (of which only the last transistor 140 is shown) into the metal bit line MOi. From there, the current flows through contact O_(i)′ and select transistor 3U and into diffusion bit line 3B_(i). From there it passes through cell R to diffusion bit line 4Bi, after which the current passes through select transistor 4L and contact E_(i)′ to metal bit line ME_(i). A second series of transistors in the YMUX (of which only the first transistor 140 is shown) connects the metal bit line ME_(i) to ground. During a read operation, the current into the array flows in a similar path.

Since each element of the current path has an inherent resistance, the current flowing into and out of the array causes voltage drops therein. This leads to both a reduced drain voltage and an increased source voltage in the array cell R. Therefore, if a specific drain to source voltage V_(DS) is required for the proper operation of the array cell R, then the higher the total voltage drop along the current path, the larger the external power supply that is required.

Moreover, both in read and in program modes, the array cell R operates in saturation with a controlled gate voltage. Therefore, the higher the source voltage, the lower the current will be. Assuming no change in the gate voltage, this increases both the read access time and the programming time.

Further, while programming an NROM cell, a higher source voltage reduces the endurance of the cell. Thus, it is desirable to minimize the voltage drops along the path into and out of the array.

In order to reduce the voltage drops on the transistors along the access path to the array, the conductance of the transistors can be improved. Increasing the channel widths of the transistors is an obvious way to achieve this goal. However, this causes two undesired consequences.

1. Slower access time. This is due to the increased parasitic capacitance along the access path to the array.

2. Larger die area. Among all the transistors in the access path to the array, the select transistors have the greatest effect on the die size. This is because there is one select transistor SEL per diffusion bit line, per block. The edge transistors 140 in the YMUX path (i.e. the transistors connecting the YMUX to the metal bit lines ME or MO), also significantly affect the die size since there is one such transistor per metal bit line ME or MO. The other transistors in the YMUX path have a relatively small effect on die size since each one controls multiple bit lines and thus, there are fewer of them.

Thus, alternative methods to reduce the resistance of the transistors in the current path without the area and timing penalties mentioned above are preferred.

In one embodiment of the present invention, the select transistors SEL are implemented as low threshold voltage (V_(T)) devices. Since a transistor's resistance is proportional to 1/(V_(gs)−V_(T)), where V_(gs) is the gate to source voltage and is fixed, the low V_(T) devices have an inherently lower resistance compared to standard transistors.

However, although the low threshold voltage devices provide the desired lower resistance, producing such devices requires additional process steps, which increase the die cost.

In an alternative embodiment of the present invention, the select transistors SEL are produced with a channel length shorter than the minimum allowed design rule for standard transistors, i.e. L_(SEL)<L_(min). The shorter channel length provides the lower resistance devices.

Applicants have realized that, if the virtual ground array is operated as described in Assignee's copending U.S. patents (U.S. Pat. No. 6,134,156, issed on Oct. 17, 2000 and entitled “Method For Initiating A Retrieval Procedure In Virtual Ground Arrays” and U.S. Pat. No. 6,128,226, issued on Oct. 3, 2000 and entitled “Method And Apparatus For Operating With a Close To Ground Signal”), the select transistors always operate as “pass transistors”, i.e., there is no case in which these transistors turn on or operate with a large V_(DS). Only due to this specific mode of operation, explained with reference to FIGS. 8. 9A and 9B, can the channel length of the select transistors SEL be shortened beyond the minimum allowed design rule for standard transistors.

Reference is now made to FIG. 8, which is similar to FIG. 7 but illustrates the initial state of the memory array. Here, the whole array is initially discharged to ground after which, two select transistors SEL (such as SEL 3U and 4L) are selected to access an array cell for a future read or program operation. Thus, the transistors are activated with V_(DS, SEL) at 0V. After the select transistors SEL are activated, power is provided to the metal bit line, such as MO_(i), associated with the drain of the accessed cell while the metal bit line, such as ME_(i), associated with the source of the accessed cell is held close to ground. The remaining metal bit lines are allowed to float.

Reference is now made to FIGS. 9A and 9B, which illustrate the voltage level during accessing of a cell on the selected metal and diffusion bit lines connected to the drain (curves 144 and 146, respectively) and to the source (curves 150 and 148, respectively) of the cell. Once the metal bit lines, MO_(i) and ME_(i) in this example, have been selected, power is provided to them (depending on the operation to be performed). For example, assume MO_(i) is driven high to about 6V for programming (curve 144) and ME_(i) is driven to ground or about 200 mV (curve 150). The voltage on the metal bit line MO_(i) rises from ground level, as indicated by curve 144.

Since the select transistor SEL 3U is already turned on, the diffusion bit line 3B_(i) tracks the rise of MO_(i). This is shown in curve 146. Thus, as can be seen in FIG. 9A, at any given time, the voltage drop V_(ds) across the select transistor SEL 3U is small. Two times, t₁ and t₂, are shown and it can be seen that the voltage drops V_(ds1) and V_(ds2) at those times are small (on the order of 0.2V). On the other side of the memory cell R, as shown in FIG. 9B, a similar situation occurs as soon as the memory cell R starts conducting current (in general, word line WLB is high before the metal bit line is driven high). However, here the diffusion bit line 4B_(i) will be the first to rise (curve 148) followed by the rise of metal bit line ME_(i) (curve 150). As in the previous case, the drain to source voltage V_(DS) across select transistor SEL 4L is small (on the order of 0.1V).

In general, select transistors SEL should be able to withstand, on their terminals, the high voltages required at erase and program operations. Thus, they should be high voltage devices. However, in the array of the present invention, since the select transistors SEL always operate as pass transistors, they do not suffer the “snapback effect” which occurs with high drain to source voltage V_(DS) and therefore, their channel length can be shortened beyond the minimum allowed design rule in order to provide a smaller resistance.

How much the select transistors can be shortened depends on the punchthrough and leakage characteristics of the transistors. For example, if array cell R is accessed and diffusion bit line 3Bi is driven high, then if, the select transistor SEL 2L leaks, an additional path from metal bit line MO_(i) to metal bit line ME_(i) may be present (from diffusion bit line 3Bi, through cell Q, through diffsion bit line 2B_(i), and through the leaky select device SEL 2B_(i). Thus, the leakage characteristics of the short SEL devices should be well controlled.

As an example, if the standard minimum channel length of a high voltage device is L_(min)=0.8μm, then a typical channel length of the shorter SEL devices is L_(SEL)=0.5μm.

It should be noted that other transistors in the access path to the array (such as YMUX transistor 140) also act as pass transistors. Thus, in accordance with a preferred embodiment of the present invention, YMUX transistor 140 can also have a short channel length. Alternatively, or in addition, YMUX transistor 140 can have a low threshold voltage in order to reduce its resistance, as described hereinabove.

The methods and apparatus disclosed herein have been described without reference to specific hardware or software. Rather, the methods and apparatus have been described in a manner sufficient to enable persons of is ordinary skill in the art to readily adapt commercially available hardware and software as may be needed to reduce any of the embodiments of the present invention to practice without undue experimentation and using conventional techniques.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described bereinabove. Rather the scope of the present invention is defined only by the claims which follow. 

What is claimed is:
 1. A virtual ground array comprising: at least one memory cell; at least one select transistor to select a group of said at least one memory cell; at least one-select transistor not in an access path to said at least one select transistor, wherein a threshold voltage level of said at least one selector transistor is lower than a threshold voltage level of said at least one non-select transistor.
 2. A virtual ground array comprising: at least one memory cell; at least one select transistor to select a group of said at least one memory cell; at least non-select transistor not in an access path to said at least one select transistor, wherein a channel of said at least one select transistor is shorter than a channel of said at least one non-select transistor.
 3. A virtual ground array comprising: at least one select transistor; at least one column select transistor to select a group of said at least one select transistor; at least one non-select transistor not in an access path to said at least one column select transistor, wherein a threshold voltage level of said at least one column select transistor is lower than a threshold level of said at least one non-select transistor.
 4. A virtual ground array comprising: at least one select transistor; at least one column select transistor to select a group of said at least one select transistor, at least one non-select transistor not in an access path to said at least one column select transistor, wherein a channel of said at least one column select transistor is shorter than a channel of said at least one non-select transistor. 